This invention relates to differential pressure measuring devices that produce an indication that varies linearly with the difference in pressure between two sources of pressure. The device of this invention utilizes a measuring diaphragm subjected to the pressure differential and deflected thereby. The deflection of the diaphragm is transmitted to a measuring beam structure that has located on its surface an electrical strain gauge to produce an electrical output signal that varies linearly with respect to the applied differential pressure. The measuring diaphragm itself is protected from the substance whose pressure is to be measured by a pair of sealing diaphragms. The sealing diaphragms in conjunction with the measuring diaphragm form a pair of cavities which are filled with an essentially noncompressible fill fluid. The differential pressure to be measured is thus applied to the measuring diaphragm through the medium of the fill fluid which is located between each seal diaphragm and one side of the measuring diaphragm.
Differential pressure measuring devices are most frequently used in the determination of flow of a fluid in a pipeline. The differential pressure to be measured in such an instance is generated by the difference in pressure existing across an orifice plate or other differential pressure producer located in the pipeline. In such applications it will be recognized that the differential pressure measuring device is, itself, subjected to the existing pressure of the fluid in the pipeline. This pressure can cause distortion in the housing of the differential pressure measuring device that if transmitted to the measuring diaphragm and its support will produce an error in the output measurement. Another problem that exists with respect to the use of such differential pressure measuring devices in a flow measuring system is that such devices are occasionally subjected to differential pressures of magnitudes beyond the range of their designed capabilities. This problem is commonly referred to as overranging and is quite serious because the resulting distortion of the measuring diaphragm may render the device unpredictably nonlinear in its response to differential pressure.